Remote sensing systems, such as radar systems used to detect the presence, position, speed and/or other characteristics of objects, are vital to both civilian and military operations. These systems utilize electromagnetic (EM) waves to detect and classify, for example, precipitation and natural/man-made objects. In operation, these systems typically transmit “beams” or signals toward targets, and process reflected return signals (or echoes) for target identification and characterization. The presence of clutter in these return signals creates a significant technical challenge in the accurate processing of these signals.
In general, clutter, or components of return signals which are not of interest, can be attributed to both stationary and moving characteristics of a given background scene. Relatively stationary clutter sources include, for example, the ground, sea and various atmospheric conditions. Moving or Doppler-varying clutter sources may include precipitation as well as generally stationary objects comprising moving components. This clutter decreases radar performance by hindering the system's ability to detect targets and/or increases the probability of a false target detection.
Clutter may also comprise moving space borne objects such as decoys which likewise inhibit discrimination processes. A particular class of space borne clutter includes intentionally (or unintentionally) deployed countermeasures used by, for example, ballistic missiles. These countermeasures may be particularly challenging to ballistic missile defense (BMD) radars. In particular, the amount of radar cross-section (RCS) masking, as well as the density of countermeasure objects, can be made very great without large engineering costs. As a result, the use of these types of countermeasure systems has increased. An exemplary scenario illustrating the use of ballistic missile countermeasures is illustrated in FIG. 1. Intentional countermeasures 14 may be deployed at various stages of the flight of threat 10 by threat 10, including during exoatmospheric flight, where anti-missile activities by a BMD system are typically desirable. As set forth above, these counter measures 14 may inhibit BMD radars from discriminating, and thus accurately tracking and/or engaging, threat 10 in flight.
Several solutions have been implemented into radar signal processing systems in an effort to reduce clutter levels and improve system performance. For example, for stationary clutter, clutter mapping algorithms have been developed which create a background map and perform constant false alarm rate (CFAR) thresholding. More particularly, return signals may be received by an antenna, amplified, down-converted and passed through detector circuitry. These signals comprise desired return (e.g. target) data as well as components comprising unwanted power from clutter sources. CFAR processing attempts to determine a threshold power above which any return can be considered to originate from a target. This threshold is set typically to achieve a desired probability of a false alarm, or false alarm rate. As unwanted clutter and interference sources may have noise levels which change both spatially and temporally, a varying threshold may be used to maintain a generally constant probability of false alarm.
Moreover, a number of methods have also been implemented to remove moving clutter from radar returns. For example, a common method includes forming a track on suspected clutter as well as on objects of interest. Discrimination of the clutter from the lethal object is performed by identifying the objects of interest based on long-term behavioral characteristics, such as by using a Bayes network. This method, however, uses large amounts of radar resources to track the decoys or other objects of interest. Another method used is to filter the clutter and the object based on their radar cross-section (RCS). However, in challenging cases, the RCS of the threatening object may be smaller than many or most of the detected fragments, rendering bulk filtering based on RCS ineffective. Because these existing techniques may require tracking of the clutter, or otherwise require long processing times to discriminate the clutter from the objects of interest, the distribution of deployment velocities for the clutter may be such that they continue to complicate the radar scene throughout the exoatmospheric phase of flight, before finally being stripped away during the endoatmospheric re-entry phase. Accordingly, it is desirable to screen out a potentially large numbers of clutter objects, with few missed detections of the lethal object (low leakage), in order to concentrate focus of attention and expend radar resources on potentially threatening principal objects. In the case of a ballistic missile, if clutter generated in the exoatmosphere is not handled before re-entry, the terminal re-entry phase is the only remaining opportunity to perform discrimination, and this may not allow sufficient time to defend against the threat.
Improved systems and methods for clutter reduction, including intentional clutter resulting from the deployment of countermeasures, in radar systems are desired.